Abrasion resistant steel plate having excellent low-temperature toughness and excellent corrosive wear resistance

ABSTRACT

An abrasion resistant steel plate which possesses excellent abrasion resistance, excellent low-temperature toughness and excellent corrosive wear resistance. The abrasion resistant steel plate includes the composition containing by mass %: 0.23% to 0.35% C, 0.05% to 1.00% Si, 0.1% to 2.0% Mn, 0.020% or less P, 0.005% or less S, 0.005% to 0.100% Al, 0.03% to 2.0% Cr, and 0.03% to 1.0% Mo in a state where DI* defined by the following formula (1) is satisfied 45 or more, and further containing remaining Fe and unavoidable impurities as a balance. The steel plate has a structure where an as-quenched martensitic phase forms a main phase and a grain size of prior austenite grains is 30 μm or less, and surface hardness of the steel plate is 450 or more at Brinel hardness HBW10/3000.

TECHNICAL FIELD

The present application relates to an abrasion resistant steel platesuitably used for parts of industrial machines, transporting machinesand the like.

BACKGROUND ART

Conventionally, with respect to parts for industrial machines,transporting machines and the like such as, for example, a power shovel,a bulldozer, a hopper, a bucket or a dump truck used in a constructionsite, a civil engineering site, a mine or the like, abrasion isgenerated due to a contact of the part with earth, sand or the like.Accordingly, in manufacturing the above-mentioned parts, a steelmaterial having excellent abrasion resistance is used for extendinglifetime of the parts. In an actual in-use environment, various statessuch as a dry state or a wet state are considered as a state of earth,sand or the like. Particularly, there may be a case where earth, sand orthe like in a wet state contain a corrosive material. In this case, thewear due to earth, sand or the like in a wet state becomes wear in anenvironment which contains the corrosive material, that is, so-calledcorrosive wear. This corrosive wear has been known as an extremelysevere wear environment. In view of the above, there has been a demandfor an abrasion resistant steel material having excellent corrosive wearresistance.

The use of these industrial machines, transporting machines and the likein a low-temperature zone of 0° C. or below is also considered.Accordingly, a steel material which is used for parts of theseindustrial machines, transporting machines and the like is requested topossess the excellent low-temperature toughness in addition to theabrasion resistance and corrosive wear resistance.

To satisfy such a request, for example, patent literature 1 proposes amethod of manufacturing a high-hardness abrasion resistant steel havingexcellent low-temperature toughness, wherein hot rolling is applied to asteel slab having the composition containing by mass %: 0.30% to 0.50%C, proper amounts of Si, Mn, Al, N, Ti, Nb and B respectively, and 0.10%to 0.50% Cr and 0.05% to 1.00% Mo, thereafter, quenching treatment isapplied to the hot rolled plate from a temperature of Ar₃ transformationpoint or above and, subsequently, the quenched plate is tempered thusobtaining high-strength abrasion resistant steel. According to thedescription of the technique described in patent literature 1, theimprovement of hardenability of the steel and the improvement oflow-temperature toughness through strengthening of grain boundaries areachieved by allowing the steel to contain a large amount of Cr and alarge amount of Mo. Further, according to the description of thetechnique described in patent literature 1, the further enhancement oflow-temperature toughness is achieved by applying tempering treatment tothe steel.

Patent literature 2 proposes a high toughness abrasion resistant steelplate which has the composition containing by mass %: 0.18% to 0.25% C,0.10% to 0.30% Si, 0.03% to 0.10% Mn, proper amounts of Nb, Al, N and Brespectively, 1.00% to 2.00% Cr, and Mo more than 0.50% to 0.80%, andexhibits excellent toughness and excellent delayed fracture resistanceafter water quenching and tempering. According to the description of atechnique described in patent literature 2, by suppressing the contentof Mn to a low level, and by allowing the steel plate to contain a largeamount of Cr and a large amount of Mo, hardenability of the steel platecan be enhanced so that predetermined hardness can be ensured and, atthe same time, toughness and delayed fracture resistance can beenhanced. Further, according to the description of the techniquedescribed in patent literature 2 further improves low-temperaturetoughness by applying tempering.

Patent literature 3 proposes a high toughness and abrasion resistantsteel which has the composition containing by mass %: 0.30% to 0.45% C,0.10% to 0.50% Si, 0.30% to 1.20% Mn, 0.50% to 1.40% Cr, 0.15% to 0.55%Mo, 0.0005% to 0.0050% B, 0.015% to 0.060% sol. Al, and proper amountsof Nb and/or Ti. According to the description of the technique describedin patent literature 3, the steel contains a large amount of Cr and alarge amount of Mo and hence, hardenability of the steel is enhancedand, at the same time, grain boundaries are strengthened thus enhancinglow-temperature toughness.

Patent literature 4 proposes a method of manufacturing an abrasionresistant steel, wherein hot-rolling is applied to steel having thecomposition containing by mass %: 0.05% to 0.40% C, 0.1% to 2.0% Cr,further, proper amounts of Si, Mn, Ti, B, Al and N respectively and,further, Cu, Ni, Mo, and V as arbitrary components at a cumulativereduction ratio of 50% or more in an austenitic non-recrystallizedtemperature range at a temperature of 900° C. or below, thereafter,quenching is applied to a hot-rolled plate from a temperature of Ar₃transformation point or above and, subsequently, the quenched plate istempered, thus abrasion resistant steel being obtained. According to thedescription of this technique, directly quenching and temperingelongated austenite grains result the tempered martensitic structurewhere prior austenite grains are elongated. The tempered martensiticstructure of the elongated grains remarkably enhances low-temperaturetoughness.

Further, patent literature 5 proposes an abrasion resistant steel platehaving excellent low-temperature toughness and having the compositioncontaining by mass %: 0.10% to 0.30% C, 0.05% to 1.0% Si, 0.1% to 2.0%Mn, 0.10% to 1.40% W, 0.0003% to 0.0020% B, 0.005% to 0.10% Ti and/or0.035% to 0.1% Al. In the description of the technique described inpatent literature 5, the abrasion resistant steel plate may furthercontain one or more kinds of elements selected from a group consistingof Cu, Ni, Cr and V. Due to such composition, it is considered that theabrasion resistant steel plate has high surface hardness and exhibitsexcellent abrasion resistance and excellent low-temperature toughness.

Further, in patent literature 6, an abrasion resistant steel platehaving excellent bending property is described. The technique describedin patent literature 6 is related to an abrasion resistant steel platehaving the composition containing by mass %: 0.05% to 0.30% C, 0.1% to1.2% Ti, and not more than 0.03% solute C, and having the structurewherein a matrix is formed of a ferrite phase and a hard phase isdispersed in the matrix. The abrasion resistant steel plate described inpatent literature 6 may further contain one or two kinds of componentsselected from a group consisting of Nb and V, one or two kinds ofcomponents selected from a group consisting of Mo and W, one or twokinds of components selected from a group consisting of Si, Mn and Cu,one or two kinds of components selected from a group consisting of Niand B, and Cr. Due to such composition, regarding the abrasion resistantsteel plate described in patent literature 6, it is considered that bothabrasion resistance against abrasion caused by earth and sand andbending property can be enhanced without inducing remarkable increase ofhardness.

CITATION LIST Patent Literature

PTL 1: JP-A-H08-41535

PTL 2: JP-A-H02-179842

PTL 3: JP-A-S61-166954

PTL 4: JP-A-2002-20837

PTL 5: JP-A-2007-92155

PTL 6: JP-A-2007-197813

SUMMARY

The abrasion resistant steel plate according to embodiments hasexcellent low temperature toughness and can be suitably used as partswhich are used in places where wear or abrasion generated due to acontact of the abrasion resistant steel plate with earth and sandcontaining water must be particularly taken into consideration.

Technical Problem

The respective techniques described in patent literatures 1 to 5 aim atthe acquisition of the steel plates having low-temperature toughness andabrasion resistance. Further, the technique described in patentliterature 6 aims at the acquisition of the steel plate having bothbending property and abrasion resistance. However, in none of thesepatent literatures, the wear in an environment which contains acorrosive material such as earth and sand in a wet state has beenstudied and hence, there exists a drawback that consideration has notbeen made sufficiently with respect to corrosive wear resistance.

Further, in the respective techniques described in patent literatures 1to 4, tempering is a requisite and hence, there exists a drawback that amanufacturing cost is increased. In the technique described in patentliterature 5, the steel plate contains W as an indispensable componentand hence, there exists a drawback that a manufacturing cost isincreased. In the technique described in patent literature 6, the mainphase is formed of ferrite and hence, there is a problem that surfacehardness is low whereby the steel plate cannot acquire sufficientabrasion resistance.

The present application has been made to overcome the above-mentioneddrawbacks of the related art, and it is an object of this disclosure toprovide an abrasion resistant steel plate which can be manufactured at alow cost, possesses excellent abrasion resistance, and has both ofexcellent low-temperature toughness and excellent corrosive wearresistance.

Solution to Problem

To achieve the above-mentioned object, the inventors made extensivestudies on the influence of various factors exerted on abrasionresistance, low-temperature toughness and corrosive wear resistance ofthe steel plate. As a result of the studies, the inventors have foundthat the corrosive wear resistance of a steel plate can be remarkablyenhanced by making the steel plate have the composition containingproper amounts of Cr and Mo as indispensable components. It is supposedthat by allowing the steel plate to contain Cr and Mo, even when thesteel plate is exposed to earth and sand in a wet state having pH in avarious range, Cr and Mo exist as an oxyacid and hence, corrosive wearis suppressed.

The inventors also have found that abrasion resistance and corrosivewear resistance against abrasion caused by earth and sand can beremarkably enhanced by maintaining surface hardness of the steel plateat a high level provided that the steel plate has the above-mentionedcomposition.

The inventors also have found that the excellent low-temperaturetoughness of the steel plate can be surely acquired while the excellentabrasion resistance being assured by allowing the steel plate to containproper amounts of Cr and Mo as indispensable components and to containproper amounts of at least C, Si, Mn, P, S, Al, Cr, Mo in a state whereDI* defined by the following formula (1) is satisfied 45 or more toenhance hardenability of the steel plate, then by making the structurewhere an as-quenched martensitic phase forms a main phase with ensuringsurface hardness of 450 or more at Brinel hardness HBW 10/3000 andfurther by making the as-quenched martensitic phase finer so that agrain size of prior austenite (γ) grains is 30 μm or less.

DI*=33.85×(0.1×C)^(0.5)×(0.7×Si+1)×(3.33×Mn+1)×(0.35×Cu+1)×(0.36×Ni+1)×(2.16×Cr+1)×(3×Mo+1)×(1.75×V+1)  (1)

(where, C, Si, Mn, Cu, Ni, Cr, Mo and V denote the contents (mass %) ofrespective elements)

The present application has been made based on the above-mentionedfindings and has been completed after further study of the findings.Aspects of embodiments of this disclosure are described below.

(1) An abrasion resistant steel plate having excellent low temperaturetoughness and excellent corrosive wear resistance, the steel platehaving the composition containing by mass %: 0.23% to 0.35% C, 0.05% to1.00% Si, 0.1% to 2.0% Mn, 0.020% or less P, 0.005% or less S, 0.005% to0.100% Al, 0.03% to 2.0% Cr, and 0.03% to 1.0% Mo in a state where DI*defined by the following formula (1) is satisfied 45 or more, andfurther containing remaining Fe and unavoidable impurities as a balance,the steel plate having a structure where an as-quenched martensiticphase forms a main phase and a grain size of prior austenite grains is30 μm or less, and surface hardness of the steel plate being 450 or moreat Brinel hardness HBW10/3000.

(Formula)

DI*=33.85×(0.1×C)^(0.5)×(0.7×Si+1)×(3.33×Mn+1)×(0.35×Cu+1)×(0.36×Ni+1)×(2.16×Cr+1)×(3×Mo+1)×(1.75×V+1)  (1)

(where, C, Si, Mn, Cu, Ni, Cr, Mo and V in the formula (1) refer to thecontents (mass %) of respective elements.)

(2) In the abrasion resistant steel plate described in (1), the steelcomposition further contains by mass % one or two or more kinds ofcomponents selected from a group consisting of 0.005% to 0.1% Nb, 0.005%to 0.1% Ti, and 0.005% to 0.1% V.

(3) In the abrasion resistant steel plate described in (1) or (2), thesteel composition further contains by mass % one or two kinds ofcomponents selected from a group consisting of 0.005% to 0.2% Sn and0.005% to 0.2% Sb.

(4) In the abrasion resistant steel plate described in any of (1) to(3), the steel composition further contains by mass % one or two or morekinds of components selected from a group consisting of 0.03% to 1.0%Cu, 0.03% to 2.0% Ni, and 0.0003% to 0.0030% B.

(5) In the abrasion resistant steel plate described in any of (1) to(4), the steel composition further contains by mass % one or two or morekinds of components selected from a group consisting of 0.0005% to0.008% REM, 0.0005% to 0.005% Ca, and 0.0005% to 0.005% Mg.

(6) In the abrasion resistant steel plate described in any of (1) to(5), wherein the content of the as-quenched martensitic phase is 98% ormore in terms of volume fraction.

Advantageous Effects

According to embodiments, it is possible to manufacture, easily and in astable manner, an abrasion resistant steel plate having especiallyexcellent corrosive wear resistance in an earth-and-sand abrasionenvironment in a wet state, having excellent low temperature toughness,and excellent abrasion resistance in a stable manner without loweringsurface hardness.

DESCRIPTION OF EMBODIMENTS

Firstly, the reasons for limiting the composition of the abrasionresistance steel plate of embodiments, which is also called “the steelplate” in this specification, are explained. In the explanation madehereinafter, mass % is simply expressed by % unless otherwise specified.

C: 0.23% to 0.35%

C is an element for increasing hardness of the steel plate and forenhancing abrasive resistance. When the content of C is less than 0.23%,the steel plate cannot acquire sufficient hardness. On the other hand,when the content of C exceeds 0.35%, weldability, low-temperaturetoughness and workability of the steel plate are lowered. Accordingly,the content of C is limited to a value which falls within a range from0.23% to 0.35%. The content of C is preferably limited to a value whichfalls within a range from 0.25% to 0.30%.

Si: 0.05% to 1.00%

Si is an effective element acting as a deoxidizing agent for moltensteel. Si is also an element which contributes to the enhancement ofstrength of the steel plate by increasing solid solution strengthening.The content of Si is set to 0.05% or more to ensure such effects. Whenthe content of Si is less than 0.05%, a deoxidizing effect cannot besufficiently acquired. On the other hand, when the content of Si exceeds1.00%, ductility and toughness of the steel plate are lowered, and thecontent of inclusions in the steel plate is increased. Accordingly, thecontent of Si is limited to a value which falls within a range from0.05% to 1.00%. The content of Si is preferably limited to a value whichfalls within a range from 0.15% to 0.45%.

Mn: 0.1% to 2.0%

Mn is an element having an action of enhancing hardenability. To ensuresuch an effect, the content of Mn is set to 0.1% or more. On the otherhand, when the content of Mn exceeds 2.0%, temper embrittlement isoccurred and weld heat-affected zone become hardened, weldability beinglowered. Accordingly, the content of Mn is limited to a value whichfalls within a range from 0.1% to 2.0%. The content of Mn is preferablylimited to a value which falls within a range from 0.4% to 1.7%. It ismore preferable that the content of Mn is limited to a value which fallswithin a range from 0.5% to 1.0%.

P: 0.020% or less

When the content of P in steel is large, lowering of low-temperaturetoughness of the steel plate is induced and hence, it is desirable thatthe content of P be as small as possible. According to embodiments, thepermissible content of P is 0.020%. The excessive reduction of thecontent of P induces the sharp rise in a refining cost. Accordingly, itis desirable to set the content of P to 0.005% or more.

S: 0.005% or less

When the content of Sin steel is large, S is precipitated as MnS. Inhigh strength steel, MnS becomes an initiation point of the occurrenceof fracture and induces deterioration of toughness of the steel plateand hence, it is desirable that the content of S be as small aspossible. According to embodiments, the permissible content of S is0.005%. Accordingly, the content of S is limited to 0.005% or less. Theexcessive reduction of the content of S induces the sharp rise of arefining cost. Accordingly, it is desirable to set the content of S to0.0005% or more.

Al: 0.005% to 0.100%

Al is an element acting as a deoxidizing agent for molten steel.Further, Al contributes for the enhancement of low-temperature toughnessdue to refining of crystal grains. To acquire such an effect, thecontent of Al is set to 0.005% or more. When the content of Al is lessthan 0.005%, such an effect cannot be sufficiently acquired. On theother hand, when the content of Al exceeds 0.100%, weldability of thesteel plate is lowered. Accordingly, the content of Al is limited to avalue which falls within a range from 0.005% to 0.100%. The content ofAl is preferably limited to a value which falls within a range from0.015% to 0.050%.

Cr: 0.03% to 2.0%

Cr has an effect of increasing hardenability. Cr has also an effect ofenhancing low-temperature toughness due to refining of a martensiticphase. Accordingly, in embodiments, Cr is an important element. Further,in a corrosive wear environment where a contact between a steel plateand earth and sand or the like in a wet state becomes a problem, Cr isdissolved as chromate ion due to an anodic reaction, and suppressescorrosion due to an inhibitor effect thus giving rise to an effect ofenhancing corrosive wear resistance of the steel plate. To acquire suchan effect, the content of Cr is set to 0.03% or more. When the contentof Cr is less than 0.03%, the steel plate cannot exhibit such an effectsufficiently. On the other hand, when the content of Cr exceeds 2.0%,weldability is lowered and a manufacturing cost is sharply increased.Accordingly, the content of Cr is limited to a value which falls withina range from 0.03% to 2.0%. The content of Cr is preferably limited to avalue which falls within a range from 0.07% to 1.0%. It is morepreferable that the content of Cr is limited to a value which fallswithin a range from 0.2% to 0.9%.

Mo: 0.03% to 1.0%

Mo has an effect of increasing hardenability. Mo has also an effect ofenhancing low-temperature toughness due to refining of a martensiticphase. Accordingly, in embodiments, Mo is an important element. Further,in a corrosive wear environment where a contact between a steel plateand earth and sand or the like in a wet state becomes a problem, Mo isdissolved as molybdate ion due to an anodic reaction, and suppressescorrosion by an inhibitor effect thus giving rise to an effect ofenhancing corrosive wear resistance. To acquire such an effect, thecontent of Mo is set to 0.03% or more. When the content of Mo is lessthan 0.03%, the steel plate cannot exhibit such an effect sufficiently.On the other hand, when the content of Mo exceeds 1.0%, weldability ofthe steel plate is lowered and a manufacturing cost is sharplyincreased. Accordingly, the content of Mo is limited to a value whichfalls within a range from 0.03% to 1.0%. The content of Mo is preferablylimited to a value which falls within a range from 0.10% to 0.50%. It ismore preferable that the content of Mo is limited to a value which fallswithin a range from 0.20% to 0.40%.

By containing Cr and Mo in a combined manner in the steel plate, it isexpected that corrosive wear resistance can be enhanced remarkably. Itis based on the estimation that Cr and Mo have different pH regionswhere Cr or Mo can exist as an oxygen acid and hence, corrosive wearcaused by earth and sand or the like in a wet state having pH in a widerange can be suppressed.

The above-mentioned components are the basic components of the steel.The abrasion resistant steel plate according to embodiments may furtheroptionally contain, in addition to the above-mentioned basic components,as an optional element or optional elements, one or two or more kinds ofcomponents selected from a group consisting of 0.005% to 0.1% Nb, 0.005%to 0.1% Ti, and 0.005% to 0.1% V, and/or one or two kinds of componentsselected from a group consisting of 0.005% to 0.2% Sn and 0.005% to 0.2%Sb, and/or one or two or more kinds of components selected from a groupconsisting of 0.03% to 1.0% Cu, 0.03% to 2.0% Ni, and 0.0003% to 0.0030%B, and/or one or two or more kinds of components selected from a groupconsisting of 0.0005% to 0.008% REM, 0.0005% to 0.005% Ca, and 0.0005%to 0.005% Mg.

One or two or more kinds of components selected from a group consistingof 0.005% to 0.1% Nb, 0.005% to 0.1% Ti, and 0.005% to 0.1% V

All of Nb, Ti and V are elements which precipitate as precipitates, andenhance toughness of steel through refining of the structure. Theabrasion resistant steel plate according to embodiments, when necessary,contains one or two or more kinds of components selected from a groupconsisting of Nb, Ti and V.

Nb is an element which precipitates as carbonitride and contributes tothe enhancement of toughness through refining of the structure. Thecontent of Nb may be set to 0.005% or more for obtaining such an effect.On the other hand, when the content of Nb exceeds 0.1%, weldability maybe lowered. When the steel contains Nb, the content of Nb is preferablylimited to a value which falls within a range from 0.005% to 0.1%. Thecontent of Nb is more preferably set to a value which falls within arange from 0.012% to 0.03% from a view point of refining of thestructure.

Ti is an element which precipitates as TiN and contributes to theenhancement of toughness through fixing solid solute N. The content ofTi is set to 0.005% or more for acquiring such an effect. On the otherhand, when the content of Ti exceeds 0.1%, coarse carbonitrideprecipitates so that toughness is lowered in some cases. When the steelcontains Ti, the content of Ti is preferably limited to a value whichfalls within a range from 0.005% to 0.1%. The content of Ti ispreferably limited to a value which falls within a range from 0.005% to0.03% from a view point of the reduction of a manufacturing cost.

V is an element which precipitates as carbonitride and contributes tothe enhancement of toughness through an effect of refining thestructure. The content of V is set to 0.005% or more for acquiring suchan effect. On the other hand, when the content of V exceeds 0.1%,weldability is lowered in some cases. Accordingly, when the steelcontains V, the content of V is preferably limited to a value whichfalls within a range from 0.005% to 0.1%.

One or Two Kinds of Components Selected from a Group Consisting of0.005% to 0.2% Sn and 0.005% to 0.2% Sb

Both Sn and Sb are elements which enhance corrosive wear resistance. Theabrasion resistant steel plate according to embodiments, when necessary,contains one or two kinds of elements selected from a group consistingof Sn and Sb.

Sn is dissolved as Sn ion due to an anodic reaction, and suppressescorrosion by an inhibiter effect thus enhancing corrosive wearresistance of a steel plate. Further, Sn forms an oxide film containingSn on a surface of the steel plate and hence, an anodic reaction and acathode reaction of the steel plate are suppressed whereby corrosivewear resistance of the steel plate is enhanced. The content of Sn is setto 0.005% or more for acquiring such an effect. On the other hand, whenthe content of Sn exceeds 0.2%, the deterioration of ductility andtoughness of the steel plate may be induced. Accordingly, when the steelcontains Sn, the content of Sn is preferably limited to a value whichfalls within a range from 0.005% to 0.2%. The content of Sn is morepreferably set to a value which falls within a range from 0.005% to 0.1%from a view point of reducing tramp elements.

Sb suppresses corrosion of a steel plate by suppressing an anodicreaction of the steel plate and also by suppressing a hydrogengeneration reaction which is a cathode reaction thus enhancing corrosivewear resistance of the steel plate. The content of Sb is set to 0.005%or more for sufficiently acquiring such an effect. On the other hand,when the content of Sb exceeds 0.2%, the deterioration of toughness ofthe steel plate may be induced. Accordingly, when the steel contains Sb,the content of Sb is preferably set to a value which falls within arange from 0.005% to 0.2%. It is more preferable that the content of Sbis set to a value which falls within a range from 0.005% to 0.1%.

One or Two or More Kinds of Components Selected from a Group Consistingof 0.03% to 1.0% Cu, 0.03% to 2.0% Ni, and 0.0003% to 0.0030% B

All of Cu, Ni and B are elements which enhance hardenability. Theabrasion resistant steel plate according to embodiments, when necessary,may contain one or two or more kinds of elements selected from a groupconsisting of Cu, Ni and B.

Cu is an element which contributes to the enhancement of hardenability.The content of Cu may be 0.03% or more for acquiring such an effect. Onthe other hand, when the content of Cu exceeds 1.0%, hot workability islowered, and a manufacturing cost also sharply rises. Accordingly, whenthe steel contains Cu, the content of Cu is preferably limited to avalue which falls within a range from 0.03% to 1.0%. The content of Cuis more preferably limited to a value which falls within a range from0.03% to 0.5% from a view point of further reduction of a manufacturingcost.

Ni is an element which contributes also to the enhancement ofhardenability and the enhancement of low-temperature toughness of thesteel plate. The content of Ni may be 0.03% or more for acquiring suchan effect. On the other hand, when the content of Ni exceeds 2.0%, amanufacturing cost may rise. When the steel contains Ni, the content ofNi is preferably limited to a value which falls within a range from0.03% to 2.0%. The content of Ni is more preferably limited to a valuewhich falls within a range from 0.03% to 0.5% from a viewpoint offurther reduction of a manufacturing cost.

B is an element which contributes to the enhancement of hardenabilitywith a small amount in steel. The content of B may be 0.0003% or morefor acquiring such an effect. On the other hand, when the content of Bexceeds 0.0030%, toughness of the steel plate may be lowered.Accordingly, when the steel contains B, the content of B is preferablylimited to a value which falls within a range from 0.0003% to 0.0030%.The content of B more preferably falls within a range from 0.0003% to0.0015% from a viewpoint of suppressing cold cracking at a welded partformed by low-heat input welding such as CO₂ welding or the like used ingeneral in welding of an abrasion resistant steel plate.

One or Two or More Kinds of Components Selected from a Group Consistingof 0.0005% to 0.008% REM, 0.0005% to 0.005% Ca, and 0.0005% to 0.005% Mg

All of REM, Ca and Mg are elements which form sulfide inclusions bycombining with S and hence, these elements are elements which suppressthe formation of MnS. The abrasion resistant steel plate according toembodiments, when necessary, contains one or two or more kinds ofcomponents selected from a group consisting of REM, Ca and Mg.

REM fixes S thus suppressing the formation of MnS which causes loweringof toughness of the steel plate. The content of REM may be 0.0005% ormore for acquiring such an effect. On the other hand, when the contentof REM exceeds 0.008%, the contents of inclusions in the steel plate areincreased so that toughness is lowered in some cases. When the steelcontains REM, the content of REM is preferably limited to a value whichfalls within a range from 0.0005% to 0.008%. The content of REM is morepreferably set to a value which falls within a range from 0.0005% to0.0020%.

Ca fixes S thus suppressing the formation of MnS which causes loweringof toughness. The content of Ca may be 0.0005% or more for acquiringsuch an effect. On the other hand, when the content of Ca exceeds0.005%, the content of inclusions in the steel is increased andtoughness may be lowered to the contrary. When the steel contains Ca,the content of Ca is preferably limited to a value which falls within arange from 0.0005% to 0.005%. The content of Ca is more preferably setto a value which falls within a range from 0.0005% to 0.0030%.

Mg fixes S thus suppressing the formation of MnS which causes loweringof toughness of the steel plate. The content of Mg may preferably be0.0005% or more for acquiring such an effect. On the other hand, whenthe content of Mg exceeds 0.005%, the content of inclusions in the steelplate is increased and toughness may be lowered to the contrary. Whenthe steel contains Mg, the content of Mg is preferably limited to avalue which falls within a range from 0.0005% to 0.005%. It is morepreferable that the content of Mg is set to a value which falls within arange from 0.0005% to 0.0040%.

The abrasion resistant steel plate according to embodiments has theabove-mentioned components within the above-mentioned rages and in astate where DI* is satisfied 45 or more. DI* is defined by the followingformula (1). In the calculation for DI*, regarding the elementsdescribed in the formula (1), elements not contained in the steel arecalculated as Zero.

DI*=33.85×(0.1×C)^(0.5)×(0.7×Si+1)×(3.33×Mn+1)×(0.35×Cu+1)×(0.36×Ni+1)×(2.16×Cr+1)×(3×Mo+1)×(1.75×V+1)  (1)

(where, C, Si, Mn, Cu, Ni, Cr, Mo and V are the contents (mass %) ofrespective elements.)

When DI* is set to less than 45, a quenching depth from a surface of thesteel plate becomes less than 10 mm and hence, a lifetime of the steelplate as the abrasion resistant steel plate is shortened. Accordingly,DI* is limited 45 or more. The range of DI* is preferably set to 75 ormore.

Remaining other than the above-mentioned compositions are Fe andunavoidable impurities as a balance.

Next, the structure and the property of the abrasion resistant steelplate of the present disclosure are explained.

The abrasion resistant steel plate according to embodiments has theabove-mentioned composition and the structure wherein an as-quenchedmartensitic phase forms a main phase and a grain size of prior austenite(γ) grains is 30 μm or less. Further, the abrasion resistant steel plateaccording to embodiments has surface hardness of 450 or more at Brinelhardness HBW 10/3000. Here, a phase which occupies 90% or more in anarea ratio is defined as “main phase”.

As-Quenched Martensitic Phase: 90% or More in Area Ratio

When the phase fraction of the as-quenched martensitic phase is lessthan 90% in an area ratio, the steel plate cannot ensure desiredhardness. Accordingly, when the area ratio is less than 90%, wearresistance of the steel plate is lowered so that desired wear resistancecannot be ensured. Further, the steel plate cannot ensure the sufficientlow-temperature toughness. Further, in tempered martensite phase, Cr andMo form carbide together with Fe when cementite is formed in tempering.Due to the formation of carbide, solute Cr and solute Mo, which areeffective to ensure corrosion resistance, are decreased. Accordingly,the martensitic phase is held in the as-quenched martensitic phase wherethe martensitic phase is not tempered. A phase fraction of theas-quenched martensitic phase is preferably set to 95% or more in arearatio, and it is more preferable that the phase fraction of theas-quenched martensitic phase is set to 98% or more in area ratio.

Grain Size of Prior Austenite (γ) Grains: 30 μm or Less

Even when the phase fraction of the as-quenched martensitic phase canensure the area ratio of 90% or more, when a grain size of prioraustenite (γ) grains becomes coarse exceeding 30 μm, the low-temperaturetoughness of the steel plate is lowered. As the grain size of prioraustenite (γ) grains, values which are obtained in accordance with JIS G0551 after microscopically observing the structure etched by a picricacid using an optical microscope (magnification: 400 times) are used.

The abrasion resistant steel plate according to embodiments having theabove-mentioned composition and structure has surface hardness of 450 ormore at Brinel hardness HBW 10/3000.

Surface Hardness: 450 or More at Brinel Hardness HBW 10/3000

When the surface hardness of steel is less than 450 at Brinel hardnessHBW 10/3000, the lifetime of the abrasion resistant steel plate becomesshort. Accordingly, the surface hardness is set to 450 or more at Brinelhardness HBW 10/3000. Brinel hardness is measured in accordance with thestipulation described in JIS Z 2243.

Next, the preferred method of manufacturing the abrasion resistant steelplate of this disclosure is explained.

The steel material having the above-mentioned composition is produced bycasting and then subjected to hot rolling without cooling when the steelmaterial holds a predetermined temperature or subjected to hot rollingafter cooling and reheating, thus manufacturing a steel plate having adesired size and a desired shape.

The method of manufacturing the steel material is not particularlylimited. It is desirable that molten steel having the above-mentionedcomposition is produced using a known refining method such as using aconverter, and a steel material such as a slab having a predeterminedsize is manufactured by a known casting method such as a continuouscasting method. It goes without saying that a steel material can bemanufactured by an ingot casting-blooming method.

Reheating Temperature: 950 to 1250° C.

When the reheating temperature is below 950° C., the deformationresistance becomes excessively high so that a rolling load becomesexcessively large whereby hot rolling may not be performed. On the otherhand, when the reheating temperature becomes high exceeding 1250° C.,the crystal grains become excessively coarse so that steel may notensure desired high toughness. Accordingly, the reheating temperature ispreferably limited to a value which falls within a range from 950 to1250° C.

The reheated steel material or the steel material which holds apredetermined temperature without being reheated is, then, subjected tohot rolling so that a steel plate having a desired size and a desiredshape is manufactured. The hot rolling condition is not particularlylimited. After the hot rolling is finished, it is preferable that directquenching treatment where the steel plate is immediately quenched isapplied to the steel plate. It is preferable that a quenching starttemperature is set to a temperature not below an Ar3 transformationpoint. To set the quenching start temperature to the Ar3 transformationpoint or higher, it is preferable that the hot rolling finishtemperature is set to 800° C. or more not below the Ar3 transformationpoint. When the hot rolling finish temperature is excessively high,there may be a case where crystal grains become coarse. Accordingly, itis preferable that the hot rolling finish temperature is set to 950° C.or below. A quenching cooling rate is not particularly limited providedthat the quenching cooling rate is equal to or higher than a coolingrate at which a martensitic phase is formed. It is desirable that thequenching cooling rate is as high as possible to prevent a martensiticphase from being self-tempered. The solute Cr and the solute Mo, whichare effective for corrosion resistance, form carbide along with Fe whencementite is formed in the self-tempering, so that the amount of soluteCr and solute Mo is reduced. The self-tempering also reduces a volumefraction of martensite. It is desirable that the quenching cooling rateis set to 65 to 75° C./s when a plate thickness is 5 to 15 mm, thequenching cooling rate is set to 40 to 55° C./s when the plate thicknessis 16 to 22 mm, the quenching cooling rate is set to 30 to 40° C./s whenthe plate thickness is 22 to 28 mm, and the quenching cooling rate isset to 20 to 30° C./s when the plate thickness is 29 to 35 mm. Further,it is preferable that the cooling stop temperature is set to 300° C. orbelow. It is more preferable that the cooling stop temperature is 200°C. or below. In this specification, “cooling rate” is a cooling rateobtained by calculating a temperature of a center portion of a steelplate by heat transfer-heat conduction calculation.

After hot rolling is finished, in place of the direct quenchingtreatment where a steel plate is immediately quenched, treatment may beperformed where the steel plate is gradually cooled by air after the hotrolling is finished (air cooling) and, thereafter, the steel plate isreheated to a predetermined heating temperature and, thereafter, thesteel plate is quenched. It is desirable that the reheating temperatureis set to a value which falls within a range from 850 to 950° C. Aquenching cooling rate after reheating is not particularly limitedprovided that the quenching cooling rate after reheating is equal to orhigher than a cooling rate at which a martensitic phase is formed. It isdesirable that the quenching cooling rate is as high as possible toprevent a martensitic phase from being self-tempered. The solute Cr andthe solute Mo, which are effective for corrosion resistance, formcarbide along with Fe when cementite is formed in the self-tempering, sothat the amount of solute Cr and solute Mo is reduced. Theself-tempering also reduces a volume fraction of martensite. It isdesirable that the quenching cooling rate is set to 65 to 75° C./s whena plate thickness is 5 to 15 mm, the quenching cooling rate is set to 40to 55° C./s when the plate thickness is 16 to 22 mm, the quenchingcooling rate is set to 30 to 40° C./s when the plate thickness is 22 to28 mm, and the quenching cooling rate is set to 20 to 30° C./s when theplate thickness is 29 to 35 mm. Further, to prevent a martensitic phasefrom being self-tempered, it is preferable that the cooling stoptemperature is set to 300° C. or below. It is more preferable that thecooling stop temperature is set to 200° C. or below.

To acquire the as-quenched martensite structure, tempering treatment isnot performed after performing the above-mentioned treatment.

Hereinafter, disclosed embodiments are further explained based onexamples.

Examples

Molten steel having the composition described in Table 1 was produced bya vacuum melting furnace, and was cast into a mold so that ingots (steelmaterial) having a weight of 150 kgf respectively were manufactured.These steel materials were reheated at heating temperatures described inTables 2 (Table 2-1, Table 2-2, and Table 2-3) and, thereafter, thesteel materials were subjected to hot rolling under conditions describedin Table 2. Then, with respect to some steel plates, direct quenchingtreatment (DQ) where quenching (direct quenching) is immediatelyperformed after hot rolling is finished was performed under conditionsdescribed in Tables 2. With respect to other steel plates, reheatingquenching treatment (RQ) where a steel plate is cooled by air after hotrolling is finished on the respective conditions described in Table 2and the steel plate is reheated at a temperature described in Tables 2and, thereafter, is quenched was performed. In the examples described inTable 2-3, cooling rates from 800° C. to 500° C. at DQ or RQ were alsoindicated. In general, with respect to an ordinary C—Mn steel, thetransformation during cooling is started at a temperature ofapproximately 800° C. and is completed at a temperature around 500° C.Therefore, a cooling rate from 800° C. to 500° C. largely influences thetransformation behavior of steel. Accordingly, the cooling rate from800° C. to 500° C. has been generally used as a representative coolingrate for estimating the transformation behavior of steel.

Specimens were sampled from the manufactured steel plates, and thespecimens were subject to an observation of the structure, a surfacehardness test, a Charpy impact test, and a corrosive wear resistancetest. The following test methods were adopted. The results of theobservation of the structure, the surface hardness test, the Charpyimpact test, and the corrosive wear resistance test are shown in Table 3(Table 3-1, Table 3-2, and Table 3-3).

(1) Structure Observation

Specimens for structure observation were sampled from manufactured steelplates at a position of ½ plate thickness of the steel plate such thatan observation surface becomes a cross section parallel to the rollingdirection. The observation surface of the specimens for structureobservation was polished and was etched by a picric acid thus exposingprior γ grains. Thereafter, the observation surfaces were observed by anoptical microscope (magnification: 400 times). Equivalent circlediameters of respective 100 views of prior γ grains were measured, anarithmetic mean was calculated based on obtained equivalent circlediameters, and the arithmetic mean was set as the prior γ grain size ofthe steel plate.

Thin film specimens (specimens for observation of structure bytransmission electron microscope) were sampled from the manufacturedsteel plates at a position of ¼ plate thickness of the steel plate inthe same way. Next, the thin film specimen was grinded and polished(mechanical polishing, electrolytic polishing) thus forming a thin film.Next, each fields of vision of the thin film were observed by atransmission electron microscope (magnification: 20000 times), a regionwhere cementite does not precipitate was recognized as a martensiticphase region, and the area of the region was measured. The area of themartensitic phase region was indicated by a ratio (%) with respect tothe whole structure, and this ratio was set as a martensitic fraction(area ratio). Also, a kind of a phase where cementite precipitates wasdetermined.

(2) Surface Hardness Test

Specimens for surface hardness measurement were sampled from themanufactured steel plates, and surface hardness HBW 10/3000 was measuredin accordance with JIS Z 2243 (1998). In the hardness measurement, atungsten hard ball having a diameter of 10 mm was used, and a weight wasset to 3000 kgf.

(3) Charpy Impact Test

V-notched specimens were sampled from manufactured steel plates at aposition of ¼ plate thickness of the steel plate, in the direction (Cdirection) perpendicular to the rolling direction, and a Charpy impacttest was performed in accordance with the stipulation of JIS Z2242(1998). Absorbed energy vE₋₄₀ (J) was obtained under the conditionof a test temperature at −40° C. The number of specimens was three foreach of the steel plates, and an arithmetic mean of the obtained valesof three specimens is respectively set as the absorbed energy vE₋₄₀ ofthe steel plate. The steel plate having the absorbed energy vE₋₄₀ of 30J or more was evaluated as the steel plate having excellent toughness.

(4) Corrosive Wear Resistance Test

Wear specimens (size: thickness of 10 mm, width of 25 mm and length of75 mm) were sampled from manufactured steel plates at a position 1 mmaway from a surface of the manufactured steel plate. These wearspecimens were mounted on a wear tester, and a wear test was carriedout.

The wear specimen was mounted on the wear tester such that the wearspecimen was perpendicular to an axis of rotation of a rotor of thetester and a surface of 25 mm×75 mm was parallel to the circumferentialtangential direction of a rotating circle, the specimen and the rotorwere covered with an outer vessel, and a wear material was introducedinto the inside of the outer vessel. As the wear material, a mixture isused where silica sand having an average grain size of 0.65 mm and anNaCl aqueous solution which was prepared such that the concentrationbecomes 15000 mass ppm were mixed together such that a weight ratiobetween silica sand and the NaCl aqueous solution becomes 3:2.

Test conditions were set such that the rotor was rotated at 600 rpm andthe outer vessel was rotated at 45 rpm. The test was finished at therevolutions of the rotor became 10800 times in total. After the test wasfinished, weights of the respective specimens were measured. Thedifference between the weight after test and the initial weight (=anamount of reduction of weight) was calculated, and a wear resistanceratio (=(reference value)/(amount of reduction of weight of specimen))was calculated using an amount of reduction of weight of the steel plateSS400 stipulated in Rolled steels for general structure, Tensilestrength 400 MPa class (JIS G3101) (conventional example) as a referencevalue. When the wear resistance ratio was 1.5 or more, the steel platewas evaluated as the steel plate “having excellent corrosive wearresistance”.

TABLE 1 Chemical Composition (mass %) Steel Nb, Cu, Ca, Num- Ti, Sn, Ni,REM, Ar3 ber C Si Mn P S sol.Al Cr Mo V Sb B Mg DI* (° C.) Remarks A0.26 0.33 1.64 0.007 0.0017 0.032 0.05 0.05 — — — — 55.3 693 withinscope of disclosed embodiments B 0.23 0.25 1.22 0.008 0.0024 0.027 0.200.10 — — — — 56.8 730 within scope of disclosed embodiments C 0.24 0.410.62 0.007 0.0019 0.025 1.10 0.10 — — Cu: — 98.0 753 within 0.01, ofscope Ni: disclosed 0.12, embodiments B: 0.0021 D 0.27 0.25 0.75 0.0070.0015 0.025 0.38 0.16 Nb: — B: — 61.6 748 within scope 0.022, 0.0009 ofdisclosed Ti: embodiments 0.014 E 0.26 0.26 0.65 0.008 0.0013 0.022 0.450.11 Nb: — B: — 53.5 762 within scope 0.025, 0.0013 of disclosed Ti:embodiments 0.017 F 0.28 0.30 0.85 0.008 0.0015 0.027 0.25 0.25 Nb: — B:— 70.8 731 within scope 0.017, 0.0006 of disclosed Ti: embodiments 0.010G 0.26 0.27 0.76 0.008 0.0015 0.027 0.40 0.15 Nb: — B: Ca: 61.9 751within scope 0.015, 0.0020 0.0022 of disclosed Ti: embodiments 0.015 H0.29 0.32 1.23 0.008 0.0018 0.023 0.10 0.06 Ti: — — REM: 51.6 715 withinscope 0.022 0.0015 of disclosed embodiments I 0.27 0.32 1.32 0.0080.0018 0.023 0.15 0.15 Nb: — — — 71.4 705 within scope 0.013, ofdisclosed Ti: embodiments 0.015 J 0.30 0.35 0.50 0.006 0.0022 0.024 0.300.65 V: — — Ca: 100.4 721 within scope 0.035 0.0021 of disclosedembodiments K 0.24 0.32 1.05 0.007 0.0027 0.021 0.12 0.32 Ti: — B: Mg:71.2 724 within scope 0.013 0.0009 0.016 of disclosed embodiments L 0.310.27 0.57 0.007 0.0015 0.023 0.76 0.11 Nb: — B: — 74.2 748 within scope0.019, 0.0025 of disclosed V: embodiments 0.016 M 0.28 0.30 1.21 0.0080.0016 0.025 0.13 0.16 Nb: — B: — 6.53 712 within scope 0.021, 0.0013 ofdisclosed Ti: 0.015 embodiments N 0.26 0.29 1.02 0.007 0.0014 0.019 0.530.25 Nb: Sb: Cu: Ca: 138.3 698 within scope 0.029, 0.066 0.24, 0.012 ofdisclosed Ti: Ni: embodiments 0.021, 0.31 V: 0.034 O 0.26 0.36 1.520.008 0.0016 0.024 0.02 — Ti: — — Ca: 43.2 708 outside scope 0.0160.0018 of disclosed embodiments P 0.29 0.35 1.42 0.007 0.0019 0.025 —0.02 V: — — Mg: 45.2 705 outside scope 0.021 0.0032 of disclosedembodiments Q 0.30 0.38 1.36 0.006 0.0021 0.029 0.01 0.02 — — Cu: — 45.7705 outside scope 0.08 of disclosed embodiments R 0.18 0.24 0.88 0.0080.0016 0.024 0.28 0.15 Nb: Sn: B: — 48.5 768 outside scope 0.015 0.0150.0022 of disclosed embodiments S 0.25 0.31 0.76 0.007 0.0017 0.021 0.090.10 Nb: Sb: Cu: REM: 38.2 755 outside scope 0.013 0.033 0.1, 0.0012 ofdisclosed Ni: embodiments 0.09 T 0.28 0.26 1.09 0.007 0.0025 0.024 0.050.27 — — — — 62.2 714 within scope of disclosed embodiments U 0.27 0.300.93 0.007 0.0019 0.028 0.43 0.19 — — — — 83.5 730 within scope ofdisclosed embodiments V 0.28 0.25 1.13 0.009 0.0029 0.022 0.52 0.13 —Sn: — — 93.6 715 within scope 0.021 of disclosed embodiments W 0.29 0.360.85 0.008 0.0021 0.031 0.75 0.11 Nb: Sn: — — 96.3 732 within scope0.014 0.067 of disclosed embodiments

TABLE 2-1 Hot Rolling DQ RQ Rolling Cooling Cooling Cooling ReheatingFinish Start Stop Heating Stop Steel Plate Temper- Temper- Temper-Cooling Temper- Temper- Temper- Plate Steel Thickness Type of atureature ature After ature ature Cooling ature Number Number (mm)Treatment* (° C.) (° C.) (° C.) Rolling (° C.) (° C.) Method (° C.) 1 A12 RQ 1120 900 — cooled by air — 900 cooled by water 150 2 A 19 RQ 1120920 — cooled by air — 910 cooled by water 170 3 A 25 DQ 1120 880 830cooled by water 150 — — — 4 A 25 DQ 1250 950 870 cooled by water 310 — —— 5 A 25 DQ 1120 980 900 cooled by water 310 — — — 6 B 12 RQ 1120 890 —cooled by air — 900 cooled by water 150 7 B 19 DQ 1120 870 850 cooled bywater 150 — — — 8 B 32 DQ 1120 890 840 cooled by water 150 — — — 9 B 32DQ 1200 970 900 cooled by water 250 — — — 10 B 32 DQ 1230 960 900 cooledby water 250 — — — 11 C 19 DQ 1050 840 810 cooled by water 150 — — — 12C 25 DQ 1050 850 800 cooled by water 130 — — — 13 C 35 DQ 1050 880 820cooled by water 100 — — — 14 D 19 RQ 1100 870 — cooled by air — 910cooled by water 170 15 D 25 RQ 1100 890 — cooled by air — 910 cooled bywater 170 16 D 35 DQ 1100 890 870 cooled by water 100 — — — 17 E 19 RQ1100 870 — cooled by air — 910 cooled by water 260 18 E 25 RQ 1100 890 —cooled by air — 910 cooled by water 160 19 F 35 DQ 1100 890 870 cooledby water 150 — — — 20 F 19 RQ 1100 870 — cooled by air — 910 cooled bywater 160 21 F 25 RQ 1100 890 — cooled by air — 910 cooled by water 16022 G 35 DQ 1100 890 870 cooled by water 150 — — — 23 G 19 RQ 1100 870 —cooled by air — 910 cooled by water 280 24 G 25 RQ 1100 890 — cooled byair — 910 cooled by water 180 25 G 35 DQ 1100 890 870 cooled by water150 — — — 26 H 6 RQ 1120 910 — cooled by air — 880 cooled by water 15027 H 19 RQ 1120 930 — cooled by air — 900 cooled by water 150 28 H 32 DQ1120 870 800 cooled by water 170 — — — 29 I 6 RQ 1120 850 — cooled byair — 950 cooled by water 150 30 I 12 RQ 1120 860 — cooled by air — 870cooled by water 150 31 I 19 DQ 1120 890 830 cooled by water 150 — — — 32J 12 RQ 1110 860 — cooled by air — 870 cooled by water 150 33 J 19 DQ1110 870 840 cooled by water 170 — — — 34 J 35 DQ 1110 880 850 cooled bywater 170 — — — 35 K 6 RQ 1120 840 — cooled by air — 930 cooled by water150 36 K 12 RQ 1120 870 — cooled by air — 900 cooled by water 150 37 K20 DQ 1120 890 830 cooled by water 180 — — — *DQ: direct quenching, RQ:reheating quenching

TABLE 2-2 Hot Rolling DQ RQ Rolling Cooling Cooling Cooling ReheatingFinish Start Stop Heating Stop Steel Plate Temper- Temper- Temper-Cooling Temper- Temper- Temper- Plate Steel Thickness Type of atureature ature after ature ature Cooling ature Number Number (mm)Treatment* (° C.) (° C.) (° C.) Rolling (° C.) (° C.) Method (° C.) 38 L20 DQ 1150 920 880 cooled by water 180 — — — 39 L 25 RQ 1150 930 —cooled by air — 900 cooled by water 150 40 L 35 DQ 1150 910 870 cooledby water 180 — — — 41 M 12 DQ 1170 900 860 cooled by water 160 — — — 42M 25 DQ 1170 920 880 cooled by water 140 — — — 43 M 35 RQ 1170 880 —cooled by air — 900 cooled by water 250 44 N 12 RQ 1080 890 — cooled byair — 930 cooled by water 160 45 N 19 DQ 1080 870 830 cooled by water100 — — — 46 N 25 DQ 1080 850 810 cooled by water 120 — — — 47 O 12 RQ1180 840 — cooled by air — 900 cooled by water 280 48 O 19 RQ 1180 930 —cooled by air — 930 cooled by water 280 49 O 30 DQ 1180 900 850 cooledby water 250 — — — 50 P 6 DQ 1150 880 840 cooled by water 250 — — — 51 P19 DQ 1150 840 820 cooled by water 250 — — — 52 P 35 DQ 1150 820 800cooled by water 250 — — — 53 Q 19 RQ 1130 930 — cooled by air — 900cooled by water 320 54 Q 25 DQ 1130 920 890 cooled by water 280 — — — 55Q 35 DQ 1130 850 830 cooled by water 280 — — — 56 R 12 RQ 1200 860 —cooled by air — 900 cooled by water 310 57 R 25 RQ 1200 890 — cooled byair — 900 cooled by water 290 58 R 35 DQ 1200 880 840 cooled by water300 — — — 59 S 6 RQ 1120 850 — cooled by air — 880 cooled by water 21060 S 19 DQ 1120 870 830 cooled by water 300 — — — 61 S 35 RQ 1120 900 —cooled by air — 850 cooled by water 210 62 T 12 RQ 1120 920 — cooled byair — 920 cooled by water 150 63 T 25 RQ 1120 900 — cooled by air — 900cooled by water 150 64 T 32 RQ 1120 880 — cooled by air — 870 cooled bywater 150 65 U 12 RQ 1180 900 — cooled by air — 890 cooled by water 15066 U 19 DQ 1180 880 850 cooled by water 150 — — — 67 U 32 RQ 1180 890 —cooled by air — 870 cooled by water 180 68 V 12 RQ 1120 870 — cooled byair — 920 cooled by water 180 69 V 25 RQ 1120 930 — cooled by air — 910cooled by water 180 70 V 32 RQ 1120 900 — cooled by air — 920 cooled bywater 180 71 W 12 DQ 1180 860 830 cooled by water 150 — — — 72 W 19 RQ1180 900 — cooled by air — 900 cooled by water 190 73 W 32 RQ 1180 910 —cooled by air — 870 cooled by water 190 *DQ: direct quenching, RQ:reheating quenching

TABLE 2-3 Cooling Hot Rolling DQ Rate in Rolling Cooling Cooling RQCooling Plate Reheating Finish Start Stop Heating Cooling by Water SteelThick- Type of Temper- Temper- Temper- Cooling Temper- Temper- Stop 800°C. → Plate Steel ness Treat- atrure ature ature After ature atureCooling Temp. 500° C. Number Number (mm) ment* (° C.) (° C.) (° C.)Rolling (° C.) (° C.) Method (° C.) (° C.) Remarks 74 A 12 RQ 1120 900 —cooled by — 900 cooled by 150 32 example air water 75 A 12 RQ 1120 900 —cooled by — 910 cooled by 150 28 example air water 76 A 12 RQ 1120 900 —cooled by — 900 cooled by 150 62 example air water 77 A 12 DQ 1120 880860 cooled by 145 — — — 65 example water 78 A 12 DQ 1120 870 850 cooledby 150 — — — 71 example water 79 A 19 RQ 1120 920 — cooled by — 910cooled by 170 19 example air water 80 A 19 RQ 1120 920 — cooled by — 900cooled by 150 19 example air water 81 A 19 DQ 1120 890 840 cooled by 150— — — 41 example water 82 A 25 DQ 1120 880 830 cooled by 150 — — — 15example water 83 A 25 RQ 1120 890 — cooled by — 900 cooled by 150 32example air water 84 B 12 RQ 1120 890 — cooled by — 900 cooled by 150 32example air water 85 B 12 RQ 1120 890 — cooled by — 900 cooled by 160 65example air water 86 B 12 DQ 1120 950 890 cooled by 150 — — — 71 examplewater 87 B 19 DQ 1120 870 850 cooled by 150 — — — 19 example water 88 B19 DQ 1120 950 890 cooled by 150 — — — 40 example water 89 B 32 DQ 1120890 840 cooled by 150 — — — 12 example water *DQ: direct quenching, RQ:reheating quenching

TABLE 3-1 Corrosive Wear Resistance Wear Structure Low- Resistance GrainSize Surface temperature Ratio Steel of Prior Martensitic HardnessToughness (Reference: 1.0 Plate Steel Austenite Fraction HBW vE₋₄₀(conventional Number Number Grains (μm) (area %) 10/3000 (° C.) example)Remarks 1 A 25 93 486 33 1.5 example 2 A 27 92 491 32 1.5 example 3 A 2791 493 31 1.5 example 4 A 28 85 432 30 1.3 comparison example 5 A 32 83430 17 1.2 comparison example 6 B 22 96 469 38 1.9 example 7 B 25 93 46834 1.9 example 8 B 26 92 459 33 1.9 example 9 B 36 92 466 17 1.9comparison example 10 B 35 94 471 14 2.0 comparison example 11 C 16 97465 39 1.9 example 12 C 18 95 469 36 2.0 example 13 C 19 93 472 34 2.1example 14 D 15 95 455 45 2.0 example 15 D 12 96 460 46 2.1 example 16 D10 94 465 50 2.3 example 17 E 15 95 470 45 2.0 example 18 E 14 96 475 462.1 example 19 F 12 94 490 52 2.4 example 20 F 16 95 470 42 2.0 example21 F 13 95 489 46 2.1 example 22 G 12 94 498 47 2.0 example 23 G 18 94470 46 2.0 example 24 G 17 93 478 45 2.1 example 25 G 15 95 498 48 2.1example 26 H 25 95 515 35 1.5 example 27 H 27 93 519 33 1.5 example 28 H28 91 521 32 1.5 example 29 I 22 96 493 33 1.6 example 30 I 24 94 503 361.6 example 31 I 25 92 505 32 1.6 example 32 J 21 97 521 38 2.0 example33 J 17 95 534 40 2.1 example 34 J 16 93 539 42 2.0 example 35 K 23 96465 36 2.1 example 36 K 20 93 470 37 2.1 example 37 K 24 92 481 34 2.1example

TABLE 3-2 Corrosive Wear Resistance Wear Structure Low- Resistance GrainSize Surface temperature Ratio Steel of Prior Martensitic HardnessToughness (Reference: 1.0 Plate Steel Austenite Fraction HBW vE₋₄₀(conventional Number Number Grains (μm) (area %) 10/3000 (° C.) example)Remarks 38 L 12 97 557 49 2.4 example 39 L 13 95 545 57 2.4 example 40 L13 93 550 52 2.4 example 41 M 11 93 508 45 1.6 example 42 M 12 94 512 421.6 example 43 M 10 92 505 45 1.5 example 44 N 13 99 490 73 2.5 example45 N 10 98 493 62 2.5 example 46 N 8 97 488 66 2.5 example 47 O 32 92482 27 0.8 comparison example 48 O 34 91 491 25 0.8 comparison example49 O 31 93 493 24 0.8 comparison example 50 P 38 95 531 17 0.9comparison example 51 P 36 92 524 22 0.9 comparison example 52 P 32 93519 24 0.9 comparison example 53 Q 33 94 521 28 1.2 comparison example54 Q 32 92 532 25 1.2 comparison example 55 Q 34 92 530 27 1.2comparison example 56 R 15 96 413 51 1.4 comparison example 57 R 16 93410 48 1.4 comparison example 58 R 16 91 409 44 1.4 comparison example59 S 22 52 420 15 1.2 comparison example 60 S 21 55 425 20 1.2comparison example 61 S 25 47 413 12 1.2 comparison example 62 T 27 94507 34 1.6 example 63 T 26 94 509 36 1.6 example 64 T 25 93 506 37 1.6example 65 U 23 96 511 37 2.1 example 66 U 26 95 510 35 2.1 example 67 U22 96 507 40 2.1 example 68 V 20 97 520 40 2.4 example 69 V 19 96 523 432.4 example 70 V 21 97 519 38 2.5 example 71 W 21 97 528 45 2.4 example72 W 17 97 531 48 2.4 example 73 W 15 96 521 51 2.4 example

TABLE 3-3 Corrosive Wear Resistance Wear Structure Low- Resistance GrainSize Surface temperature Ratio Steel of Prior Martensitic HardnessToughness (Reference: 1.0 Plate Steel Austenite Fraction HBW vE₋₄₀(conventional Number Number Grains (μm) (area %) 10/3000 (° C.) example)Remarks 74 A 25 93 486 33 1.5 example 75 A 27 94 493 34 1.5 example 76 A26 97 500 32 1.6 example 77 A 25 98 501 31 1.7 example 78 A 26 99 504 321.8 example 79 A 27 92 491 32 1.5 example 80 A 27 92 492 33 1.5 example81 A 26 96 498 34 1.6 example 82 A 27 91 493 31 1.5 example 83 A 26 95496 30 1.7 example 84 B 22 96 469 38 1.9 example 85 B 21 98 473 39 2.0example 86 B 20 99 477 38 2.1 example 87 B 25 93 468 34 1.9 example 88 B25 96 472 36 2.0 example 89 B 26 92 459 33 1.9 example

All of the examples according to disclosed embodiments exhibit highsurface hardness of 450 or more in HBW 10/3000, excellentlow-temperature toughness of vE₋₄₀ of 30 J or more, and excellentcorrosive wear resistance of the wear resistance ratio of 1.5 or more.Moreover, the steel plate cooled with higher cooling rate has a highermartensitic fraction. Particularly, the steel plate having martensiticfraction of 98% or more exhibits excellent corrosive wear resistance inparticular, as compared with the steel plate having martensitic fractionof less than 98% and having same composition. On the other hand, thecomparative examples which fall outside the scope of disclosedembodiments exhibit lowering of surface hardness, lowering oflow-temperature toughness, lowering of corrosive wear resistance orlowering of two or more of these properties.

1-6. (canceled)
 7. An abrasion resistant steel plate having excellent low temperature toughness and excellent corrosive wear resistance, the steel plate having a composition comprising: 0.23% to 0.35% C, by mass %; 0.05% to 1.00% Si, by mass %; 0.1% to 2.0% Mn, by mass %; 0.020% or less P, by mass %; 0.005% or less S, by mass %; 0.005% to 0.100% Al, by mass %; 0.03% to 2.0% Cr, by mass %; 0.03% to 1.0% Mo, by mass %, and in an amount where DI* defined by the following formula (1) is 45 or more; DI*=33.85×(0.1×C)^(0.5)×(0.7×Si+1)×(3.33×Mn+1)×(0.35×Cu+1)×(0.36×Ni+1)×(2.16×Cr+1)×(3×Mo+1)×(1.75×V+1)  (1) where C, Si, Mn, Cu, Ni, Cr, Mo and V in the formula (1) refer to the contents (mass %) of respective elements; and remaining Fe and unavoidable impurities as a balance, wherein the steel plate having a structure where an as-quenched martensitic phase forms a main phase and a grain size of prior austenite grains is in the range of 30 μm or less, and a surface hardness of the steel plate is in the range of 450 or more at Brinel hardness HBW10/3000.
 8. The abrasion resistant steel plate according to claim 7, wherein the steel composition further comprises at least one component selected from the group consisting of 0.005% to 0.1% Nb, by mass %, 0.005% to 0.1% Ti, by mass %, and 0.005% to 0.1% V, by mass %.
 9. The abrasion resistant steel plate according to claim 7, wherein the steel composition further comprises at least one component selected from the group consisting of 0.005% to 0.2% Sn, by mass %, and 0.005% to 0.2% Sb, by mass %.
 10. The abrasion resistant steel plate according to claim 8, wherein the steel composition further comprises at least one component selected from the group consisting of 0.005% to 0.2% Sn, by mass %, and 0.005% to 0.2% Sb, by mass %.
 11. The abrasion resistant steel plate according to claim 7, wherein the steel composition further comprises at least one component selected from the group consisting of 0.03% to 1.0% Cu, by mass %, 0.03% to 2.0% Ni, by mass %, and 0.0003% to 0.0030% B, by mass %.
 12. The abrasion resistant steel plate according to claim 8, wherein the steel composition further comprises at least one component selected from the group consisting of 0.03% to 1.0% Cu, by mass %, 0.03% to 2.0% Ni, by mass %, and 0.0003% to 0.0030% B, by mass %.
 13. The abrasion resistant steel plate according to claim 9, wherein the steel composition further comprises at least one component selected from the group consisting of 0.03% to 1.0% Cu, by mass %, 0.03% to 2.0% Ni, by mass %, and 0.0003% to 0.0030% B, by mass %.
 14. The abrasion resistant steel plate according to claim 10, wherein the steel composition further comprises at least one component selected from the group consisting of 0.03% to 1.0% Cu, by mass %, 0.03% to 2.0% Ni, by mass %, and 0.0003% to 0.0030% B, by mass %.
 15. The abrasion resistant steel plate according to claim 7, wherein the steel composition further comprises at least one component selected from the group consisting of 0.0005% to 0.008% REM, by mass %, 0.0005% to 0.005% Ca, by mass %, and 0.0005% to 0.005% Mg, by mass %.
 16. The abrasion resistant steel plate according to claim 8, wherein the steel composition further comprises at least one component selected from the group consisting of 0.0005% to 0.008% REM, by mass %, 0.0005% to 0.005% Ca, by mass %, and 0.0005% to 0.005% Mg, by mass %.
 17. The abrasion resistant steel plate according to claim 9, wherein the steel composition further comprises at least one component selected from the group consisting of 0.0005% to 0.008% REM, by mass %, 0.0005% to 0.005% Ca, by mass %, and 0.0005% to 0.005% Mg, by mass %.
 18. The abrasion resistant steel plate according to claim 10, wherein the steel composition further comprises at least one component selected from the group consisting of 0.0005% to 0.008% REM, by mass %, 0.0005% to 0.005% Ca, by mass %, and 0.0005% to 0.005% Mg, by mass %.
 19. The abrasion resistant steel plate according to claim 11, wherein the steel composition further comprises at least one component selected from the group consisting of 0.0005% to 0.008% REM, by mass %, 0.0005% to 0.005% Ca, by mass %, and 0.0005% to 0.005% Mg, by mass %.
 20. The abrasion resistant steel plate according to claim 12, wherein the steel composition further comprises at least one component selected from the group consisting of 0.0005% to 0.008% REM, by mass %, 0.0005% to 0.005% Ca, by mass %, and 0.0005% to 0.005% Mg, by mass %.
 21. The abrasion resistant steel plate according to claim 13, wherein the steel composition further comprises at least one component selected from the group consisting of 0.0005% to 0.008% REM, by mass %, 0.0005% to 0.005% Ca, by mass %, and 0.0005% to 0.005% Mg, by mass %.
 22. The abrasion resistant steel plate according to claim 14, wherein the steel composition further comprises at least one component selected from the group consisting of 0.0005% to 0.008% REM, by mass %, 0.0005% to 0.005% Ca, by mass %, and 0.0005% to 0.005% Mg, by mass %.
 23. The abrasion resistant steel plate according to claim 7, wherein the content of the as-quenched martensitic phase is in the range of 98% or more in terms of volume fraction.
 24. The abrasion resistant steel plate according to claim 8, wherein the content of the as-quenched martensitic phase is in the range of 98% or more in terms of volume fraction.
 25. The abrasion resistant steel plate according to claim 11, wherein the content of the as-quenched martensitic phase is in the range of 98% or more in terms of volume fraction.
 26. The abrasion resistant steel plate according to claim 15, wherein the content of the as-quenched martensitic phase is in the range of 98% or more in terms of volume fraction.
 27. The abrasion resistant steel plate according to claim 22, wherein the content of the as-quenched martensitic phase is in the range of 98% or more in terms of volume fraction. 